Reject @bean method with void return type

Closes gh-31007

Author: jhoeller

spring-context/src/main/java/org/springframework/context/annotation/BeanMethod.java

@@ -42,8 +42,13 @@ final class BeanMethod extends ConfigurationMethod {
 
 	@Override
 	public void validate(ProblemReporter problemReporter) {
+		if ("void".equals(getMetadata().getReturnTypeName())) {
+			// declared as void: potential misuse of @Bean, maybe meant as init method instead?
+			problemReporter.error(new VoidDeclaredMethodError());
+		}
+
 		if (getMetadata().isStatic()) {
-			// static @Bean methods have no constraints to validate -> return immediately
+			// static @Bean methods have no further constraints to validate -> return immediately
 			return;
 		}
 
@@ -71,6 +76,15 @@ public String toString() {
 	}
 
 
+	private class VoidDeclaredMethodError extends Problem {
+
+		VoidDeclaredMethodError() {
+			super("@Bean method '%s' must not be declared as void; change the method's return type or its annotation."
+					.formatted(getMetadata().getMethodName()), getResourceLocation());
+		}
+	}
+
+
 	private class NonOverridableMethodError extends Problem {
 
 		NonOverridableMethodError() {

spring-context/src/test/java/org/springframework/context/annotation/configuration/ConfigurationClassProcessingTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -143,6 +143,12 @@ void finalBeanMethod() {
 				initBeanFactory(ConfigWithFinalBean.class));
 	}
 
+	@Test  // gh-31007
+	void voidBeanMethod() {
+		assertThatExceptionOfType(BeanDefinitionParsingException.class).isThrownBy(() ->
+				initBeanFactory(ConfigWithVoidBean.class));
+	}
+
 	@Test
 	void simplestPossibleConfig() {
 		BeanFactory factory = initBeanFactory(SimplestPossibleConfig.class);
@@ -432,6 +438,14 @@ static class ConfigWithFinalBean {
 	}
 
 
+	@Configuration
+	static class ConfigWithVoidBean {
+
+		public @Bean void testBean() {
+		}
+	}
+
+
 	@Configuration
 	static class SimplestPossibleConfig {